First Group: Background
Hitherto, liquid crystal displays have eagerly been researched and developed. In particular, after an entrance of liquid crystal displays for large-sized televisions on the stage, the researches and developments have been more actively made.
As the material of pixel electrodes of the liquid crystal displays, ITO (indium tin oxide) is generally used. This is because ITO is excellent in electroconductivity and transparency, and can easily be etched with a strong acid (aqua regia or a hydrochloric acid-based etchant).
However, ITO formed into a film on a large-sized substrate by sputtering is a crystalline film. Thus, the state of the crystal is variously changed depending on the temperature of the substrate, the state of the atmosphere gas or plasma density, or the like so that a crystalline film and an amorphous film may be present to be mixed on the same substrate. This mixture frequently causes problems that etching failures (electric conduction to an adjacent electrode, the narrowing of the pixel electrode by overetching, pixel inferiority based on etching residues, and so on) are generated.
In order to solve the problems caused when ITO is etched, for example, Patent Document 1 discloses a method of making an ITO pixel electrode film amorphous, thereby making large the etching rate of ITO/Al by action of an etching solution of an HCl—HNO3—H2O-based so as to overcome elution of Al at the time of the etching.
About a target of ITO, black nodules are generated on the surface of the target when a film is continuously formed over a long time, so that over discharge is caused or foreign substances are generated to cause pixel inferiority. These also become problems with frequency.
Thus, the following method has been investigated: a method of adding water or hydrogen to sputtering gas when ITO is formed into a film form so as to form a film of ITO in an amorphous state, etching this film of ITO, and then heating the film to crystallize the film. However, the addition of water or hydrogen at the time of the film-formation frequently causes a decline in the adhesiveness to the underlying substrate, or causes the surface of ITO to be reduced, thereby generating a large amount of nodules.
In order to solve these problems, instead of ITO, IZO ((registered trade name) manufactured by Idemitsu Kosan Co., Ltd.: indium zinc oxide) is used. The IZO is rich in usefulness as follows: the IZO can be made into the form of a substantially completely amorphous film. The IZO can be etched with an etchant based on an oxalic acid, which is a weak acid, and can also be etched with a mixed acid of phosphoric acid, acetic acid and nitric acid, an aqueous solution of cerium (IV) ammonium nitrate, or the like. The IZO can be etched with a weaker acid than ITO. A target made of the IZO makes the generation of nodules less when the target is sputtered, and makes it possible to restrain the generation of foreign substances. Thus, it can be mentioned that the IZO is a useful target.
As a target containing the IZO, for example, Patent Document 2, which will be listed up later, discloses a target made of a sintered body of an oxide containing a hexagonal lamellar compound represented by the general formula In2O3(ZnO) wherein m=2 to 20. According to this target, a transparent electroconductive film excellent in moisture resistance (endurance) can be formed.
Furthermore, in connection with a target containing the IZO, for example, Patent Document 3, which will be listed up later, discloses a process of: painting, onto a substrate, a coating solution prepared by dissolving an indium compound and a zinc compound in the presence of an alkanolamine; firing the resultant; and then subjecting the fired product to reducing treatment, thereby producing a transparent electroconductive film. According to this transparent electroconductive film producing process, a transparent electroconductive film excellent in moisture resistance (endurance) can be obtained.
For example, Patent Document 4, which will be listed up later, discloses, as a method for etching a transparent electroconductive film containing the IZO, a method for producing a liquid crystal display, wherein a transparent electroconductive film comprising In2O3—ZnO is etched with a solution of oxalic acid in water, thereby forming a pixel electrode. According to this liquid crystal display producing process, a pixel electrode pattern can easily be formed since the oxalic acid solution is used to perform the etching. Therefore, the yield can be improved.
Second Group: Background
Liquid crystal displays have characteristics that the power consumption thereof is low and the displays can easily attain full-color display, and other characteristics. Thus, out of flat displays, the liquid crystal displays are promising; in recent years, developments for making the size of their display screens large have been actively made. In particular, about an active matrix-based liquid crystal flat display wherein α-SiTFTs (thin film transistors) or p-SiTFTs are arranged, as switching elements for respective pixels, into a matrix form, and the pixels are each driven, the contrast ratio thereof does not deteriorate even if the display is made highly definited so as to have 800×600 pixels or more. Thus, attention has been paid to the liquid crystal flat display as a flat display for high performance color display. In such an active matrix-based liquid crystal flat display, a transparent electrode such as ITO (indium tin oxide) is used as its pixel electrode, and an Al-based alloy thin film is used as its source electrode in many cases. This is because ITO has a low sheet resistance and a high permeability and Al can easily be patterned and further Al has a low resistance and a high adhesiveness.
FIG. 1 is a view illustrating a sectional view of a vicinity of an amorphous-SiTFT at a stage when the formation of a pattern of a pixel electrode is finished in the process of producing a liquid crystal flat display according to the present invention. However, any display in the prior art has the same basic structure except the material of its pixel electrode; thus, the display will be described with reference to this.
In FIG. 1, a pattern of a gate electrode 2 is formed on a light-transmissible glass substrate 1, and next plasma CVD (chemical vapor deposition) is used to form a SiN gate insulation film 3, an amorphous Si:H (i) film 4, a channel protection film 5, and an amorphous Si:H(n) film 6 continuously into desired form and patterns.
Furthermore, a metal film made mainly of Al is deposited by vacuum deposition or sputtering, and then a pattern of a source electrode 7 and a pattern 8 of a drain electrode 8 are each formed by photolithographic technique. In this way, an α-SiTFT element section is completed.
An ITO film is deposited onto this section by sputtering, and photolithographic technique is used to form a pattern of a pixel electrode 9 electrically connected to the source electrode 7. The reason why the ITO film is deposited after the Al film is deposited is that a deterioration in electrical contact property between the amorphous-Si:H 4 (and 6) and the source and drain electrodes 7 and 8 is not caused. Al is inexpensive and is low in specific resistance, and is an essential material for preventing a decline in the display performance of the liquid crystal display, the decline resulting from an increase in the resistance of the wiring of the gate electrode 2 and the wiring of the source/drain electrodes 7 and 8.
In the above-mentioned production process, when the patterns of the source electrode 7 and the drain electrode 8 made mainly of Al are formed and then the ITO pixel electrode 9 is worked into the pattern with an HCl—HNO3—H2O-based etching solution, a situation that the Al patterns elute out at the time of the end of the working may be frequently caused.
This results from the fact that Al also has a nature that Al is dissolved in the HCl—HNO3—H2O-based etching solution, which is an ITO etching solution. HNO3 in the etching solution forms a thin Al oxide film on the surface of Al, and thus HNO3 is added thereto in order to prevent elution of Al. However, it appears that if the time for etching the ITO film is long or defects portions, such as impurities or foreign substances incorporated during the deposition of Al, are present in the Al film, the above-mentioned Al-oxidizing effect does not act sufficiently by local cell reaction.
In order to prevent such Al elution, it is known that an ITO film is made amorphous, thereby increasing the ITO/Al etching rate to any HCl—HNO3—H2O-based etching solution. This matter is described in Patent Document 1, which will be listed up later. The etching rate ratio is a ratio between etching speeds.
However, even if the ITO film is made amorphous, elution of Al is not completely prevented since an HCl—HNO3—H2O-based etching solution is used. Thus, a highly definited liquid crystal display is not easily realized. As an invention made in light of such a problem, Patent Document 2, which will be listed up later, is known. This Patent Document 5 suggests that patterning of a transparent electrode and a pixel electrode 9, on a pattern of each of an Al gate electrode, and source/drain electrodes, is made easy by use of an oxalic acid-based etching solution, and a transparent electrode having a composition made of indium oxide-zinc oxide is used in order to provide a process for producing a highly definited liquid crystal display.
According to such a structure, it is known that contact resistance is generated between the line of the Al gate and the transparent electrode, and between the Al source/drain electrodes and the pixel electrode. In general, the Al lines are usually covered with a barrier metal such as Ti, Cr or Mo. Such a barrier metal is described in Patent Documents 6, 7, 8 and 9, which will be listed up later.
Third Group: Background
The matrix-based liquid crystal display is usually composed of a TFT array substrate, a counter substrate, and a display material, such as liquid crystal, sandwiched between the TFT array substrate and the counter substrate. In the matrix-based liquid crystal display, a voltage is applied to this display material in selected individual pixels, thereby displaying a given image.
On the TFT array substrate are formed a semiconductor thin film (hereinafter, referred to merely as a semiconductor film) and so on while on the counter substrate are formed, a counter electrode, a color filter, a black matrix, and so on. Such a liquid crystal display (hereinafter, simply referred to as a LCD), wherein the TFT array substrate and so on are used, will be called a TFT-LCD hereinafter.
In the above-mentioned TFT array substrate, for each element, a TFT composed of a gate electrode, a source electrode, a drain electrode and a semiconductor film, and at least a pixel electrode are formed into an array form on an insulating substrate made of glass or the like. Besides, an alignment film, a storage capacitor, and so on are formed if necessary. Between the pixels are disposed signal lines such as gate wirings and source wirings. The gate wirings and the source wirings, the numbers of which are each several, are disposed on the insulating substrate so as to cross at right angles. In this way, a display area is formed. Furthermore, an input terminal, a driving circuit for driving each of the TFTS, and so on are formed, correspondingly to each of the signal lines, outside this display area.
In order to produce such a liquid crystal display wherein a TFT array is used, gates and sources/drains included in TFTs and other common wirings are produced into an array form so as to form a display area. Input terminals, prewirings, driving circuits, and others are arranged in the vicinity of this display area.
For reference, in the present patent, a gate electrode and a gate wiring are together denoted as a gate, and a source electrode and a source wiring are together denoted as a source. A drain electrode may be denoted simply as a drain in some cases. A source and a drain are together denoted as a source/drain. In the patent, a first conductor is specifically the gate, source and drain, and a second conductor is the above-mentioned pixel electrode.
It is also preferred to arrange, on the insulating substrate of the TFT array substrate, an electroconductive thin film (hereinafter, referred to merely as an electroconductive film) or an insulating thin film (hereinafter, referred to merely as an insulation film), if necessary, in order to exhibit a given function. On the other hand, on the counter substrate are formed the counter electrode, the color filter and the black matrix, as described above.
After the TFT array substrate and the counter electrode are produced as described above, a given gap is made between the two substrates in order to inject a liquid crystal material therebetween. In this state, the two substrates are stuck to each other at their peripheries, and then a liquid crystal material is injected into the gap between these substrates to produce a TFT-LCD.
Known thin film techniques are used to form various semiconductor elements and others onto the TFT array substrate or the counter substrate used in the TFT-LCD. In order to produce, for example, semiconductor elements of a TFT array substrate, the following are formed on an insulating substrate, as described above: a gate, a source and a drain (these being referred to as the first conductor hereinafter), a semiconductor film, an insulation film, a pixel electrode (hereinafter, referred to as the second conductor), and so on. In general, at this time, the second conductor and the first conductor do not contact each other directly since the second conductor is formed on the insulation film formed on the first conductor. For this reason, a contact hole is made in the insulation film in order to attain electrical connection between the first and second conductors. In other words, the first and second conductors are electrically connected to each other through the contact hole.
It has been considered that the material of the first conductor, in particular, the material of the gate wiring and the source wiring therein is desirably pure Al or Al alloy from the viewpoint of its properties and the process in order to prevent delay of signal transmission as TFT-LCDs have been made large-sized and highly definited. This is because the pure Al or Al alloy has a low electric resistance.
However, it has not been considered that in the case of using, as the material of a transparent pixel electrode (second conductor), ITO (indium tin oxide), IZO ((registered trade name), Idemitsu Kosan Co., Ltd.: indium zinc oxide) or the like, it is not necessarily preferred to use, as the material of the first conductor, the pure Al or Al alloy for the following reason: if connection, which may be referred to as contact hereinafter, between the first and second conductors is attained, the contact resistance thereof becomes a very high value of 1×E10 to 1×E12Ω so that a favorable contact property cannot be obtained.
It has been therefore considered that it is difficult to obtain a favorable contact resistance in any TFT array substrate having a structure wherein a first conductor made of pure Al or Al alloy directly contacts a second conductor made of a transparent electroconductive film of ITO, IZO or the like through a contact hole made in an insulation film.
As the method for solving this problem, various methods have been hitherto investigated in order to obtain a favorable contact between the first and second conductors. For example, Patent Document 10, which will be listed up later, discloses a thin film transistor array substrate having a bi-layered structure composed of an Al layer comprising Al and an electroconductive metal layer which is not easily oxidized, wherein a pixel electrode is connected to a metal layer of a source electrode through a contact hole made in a passivation film. In the case that the pixel electrode is connected to the metal layer in this way, electrical contact between the source electrode and the pixel electrode becomes favorable.
Patent Document 11, which will be listed up later, discloses a process for producing an active matrix substrate, wherein a metal layer is formed into a desired thickness inside a contact hole over a drain electrode so as to reduce effectively a difference in level based on the contact hole, whereby the process for the production is made simple and a pixel electrode is not easily cut.
Patent Document 12, which will be listed up later, a thin film transistor array having a bi-layered structure drain electrode composed of a lower layer made of a metal capable of forming silicide and an upper layer made of copper, wherein a pixel electrode is connected to the upper layer of the drain electrode through a contact hole made in a protective layer. In the case that the pixel electrode is connected to the upper layer, which is made of copper, in this way, the resistance of the drain electrode is low in the thin film transistor array so that favorable electrical contact can be maintained; simultaneously, a voltage can be applied to a liquid crystal with a high efficiency.
Patent Document 13, which will be listed up later, discloses a thin film transistor having a first electrode of a bi-layered structure composed of a lower layer made of pure Al or Al alloy, and an upper layer made of pure Al or Al alloy to which at least one element selected from N, O, Si and C is added, wherein a second electrode is electrically connected to the upper layer of the first electrode.
In the source electrode or the gate electrode in the substrate in each of Patent Documents 10 to 13, which will be listed up later, a portion connected to the pixel electrode has a bi-layered structure (for example, the source electrode in Patent Document 10 has a bi-layered structure wherein Cr, Ti, Mo, Cu, Ni or the like is formed into a film on a pure Al or Al alloy.).
Fourth Group: Background
Liquid crystal displays have characteristics that the power consumption thereof is low and the displays can easily attain full-color display, and other characteristics. Thus, out of flat displays, the liquid crystal displays are promising; in recent years, liquid crystal displays of large sized screen are developed actively. In particular, about an active matrix-based liquid crystal flat display wherein α-SiTFTs or p-SiTFTs are arranged, as switching elements for respective pixels, into a matrix form, and the pixels are each driven, the contrast ratio thereof is not deteriorated even if the display is made highly definited so as to have 800×600 pixels or more. Thus, attention has been paid to the liquid crystal flat display as a flat display for high performance color display. In such an active matrix-based liquid crystal flat display, a transparent electrode such as ITO (indium tin oxide) is used as its pixel electrode, and an Al-based alloy thin film is used as its source electrode in many cases. This is because ITO has a high sheet resistance and Al can easily be patterned and further Al has a low resistance and a high adhesiveness.
FIG. 10 is a view illustrating a sectional view of a vicinity of an α-SiTFT at a stage when the formation of a pattern of a pixel electrode is finished in the process of producing a liquid crystal flat display according to the present invention. FIG. 11 is a view illustrating a sectional view of a vicinity of an α-SiTFT at a stage when the formation of a pattern of a pixel electrode is finished in the process of producing a liquid crystal flat display in the prior art.
In FIG. 11, a pattern of a gate electrode 3002 is formed on a light-transmissible glass substrate 3001, and next plasma CVD is used to form a gate insulation film 3003 made of SiN, an α-Si:H(i) film 3004, and a channel protection film 3005 made of SiN consequtively. Next, the channel protection film 3005 is made into a desired form and pattern by etching. Furthermore, an α-Si:H(n) film 3006 and a metal film made mainly of Al are deposited by CVD, vacuum deposition or sputtering, and then (a pattern of) a source electrode 3007 and (a pattern) of a drain electrode 3008 are formed by photolithographic technique, and further the α-Si:H(n) film 3006 and the α-Si:H(i) film 3004 are finished. In this way, an α-SiTFT element section is completed. If necessary, a transparent resin resist 3010 is deposited and a contact hole 3012 is made.
An ITO film is deposited onto this section by sputtering, and then photolithographic technique is used to form a pattern of a pixel electrode 3009 electrically connected to the source electrode 3007. At this time, the gate electrode 3002 is made into a laminated film of Al/Cr (or Al/Mo or Al/Ti) and the source electrode 3007 and the drain electrode 3008 are each made into a three-layer-laminated film of Cr/Al/Cr (or Mo/Al/Mo or Ti/Al/Ti). A purpose of this is to cause electrical contact property between the ITO film and the gate, source and drain electrodes 3002, 3007 and 3008 not to deteriorate. Al is inexpensive and is low in specific resistance, and is an essential material for preventing a decline in the display performance of the liquid crystal display, the decline resulting from an increase in the wiring resistances of the gate, source electrode and drain electrodes 3002, 3007 and 3008.
The reason why the ITO film is deposited after the Al film is deposited is not to deteriorate electrical contact property between α-Si:H(i) and α-Si:H(n) films 3004 and 3006 and the source and drain electrodes 3007 and 3008. This Al is inexpensive and low in specific resistance, as described above.
In the case of working, in the above-mentioned production process, the pixel electrode pattern 3009, which is made of ITO, with an HCl—HNO3—H2O-based etching solution after the formation of the patterns of the source electrode 3007 and the drain electrode 3008, which are made mainly of Al, an accident that the Al patterns elute out at the time of the end of the manufacturing process may be frequently caused.
This results from the fact that Al originally has a nature that Al is dissolved in the HCl—HNO3—H2-based etching solution, which is an etching solution for etching ITO. HNO3 in the etching solution forms a thin Al oxide film on the surface of Al, and thus HNO3 is added thereto in order to prevent elution of Al. However, it is considered that if the time for etching the ITO film is long or defects portions, such as impurities or foreign substances incorporated during the deposition of Al, are present in the Al film, the above-mentioned Al-oxidizing effect does not act sufficiently by local cell reaction.
In order to prevent such Al elution, it is known that an ITO film is made amorphous, thereby increasing the ITO/Al etching rate by action of any HCl—HNO3—H2O-based etching solution. This matter is described in Patent Document 1, which will be listed up later.
However, even if the ITO film is made amorphous, elution of Al is not completely prevented since an HCl—HNO3—H2O-based etching solution is used. Thus, a highly definited liquid crystal display is not easily realized.
An invention made in light of the above-mentioned problem is described in Patent Document 4, which will be listed up. This Patent Document 4 describes that patterning of a transparent electrode and a pixel electrode on patterns of an Al gate and source/drain electrodes is made easy by use of an oxalic acid-based etching solution. Furthermore, Patent Document 4 suggests that a transparent electrode having a composition composed of indium oxide-zinc oxide is used in order to provide a process for producing a highly definited liquid crystal display.
When such a structure is adopted, it is generally known that contact resistance is generated between the line of the Al gate and the transparent electrode, and between the Al source/drain electrodes and the pixel electrode. The Al lines are usually covered with a barrier metal such as Ti, Cr or Mo. Such a structure is described in Patent Documents 6, 7, 8 and 9, which will be listed up later.
In the case of using Al wirings in this way, it is unavoidable to use a barrier metal; it is therefore necessary to form the barrier metal into a film and etch the film. As a result, it is feared that the production process is made complicated.
The use of alloy wherein various metals are added to Al is reported. However, it is presumed that it is very difficult to make the above-mentioned contact resistance small. This is because an oxide film is formed on the surface of Al itself. This oxide film is an insulator, and it appears that this insulator makes the contact resistance large. Such a description is included in Patent Documents 14, 15 and 16, which will be listed up.
In order to decrease the contact resistance based on this oxide film, suggested is a method of forming a metal thin film made of In, Zn or the like on an Al thin film. According to this method, the contact resistance becomes small but it becomes necessary that this thin film is formed. Moreover, there arise a problem that the transmittance of the pixel electrode declines, and other problems. Such a description is included in Patent Document 17, which will be listed up later.
Patent Document 1: Japanese Patent Application Laid Open (JP-A) No. 63-184726
Patent Document 2: JP-A No. 6-234565
Patent Document 3: JP-A No. 6-187832
Patent Document 4: JP-A No. 11-264995
Patent Document 5: JP-A No. 11-264996
Patent Document 6: JP-A No. 10-65174
Patent Document 7: JP-A No. 11-184195
Patent Document 8: JP-A No. 11-258625
Patent Document 9: JP-A No. 11-253976
Patent Document 10: JP-A No. 4-253342
Patent Document 11: JP-A No. 4-305627
Patent Document 12: JP-A No. 8-18058
Patent Document 13: JP-A No. 11-284195
Patent Document 14: JP-A No. 7-45555
Patent Document 15: JP-A No. 7-301705
Patent Document 16: JP-A No. 1-289140
Patent Document 17: JP-A No. 2003-017706 (Japanese Patent Application No. 2001-200710)